1. Technical Field
The present disclosure generally relates to a method of generating an initializing value for a sequence of reference signals and a base station using the same.
2. Related Art
In the advent of a next release of a 3rd Generation Partnership Project (3GPP) specification in which Coordinated Multi-Point transmission/reception (CoMP) will be further supported, numerous challenges are still yet to be solved as well as new requirements are being posed to accommodate new additions to the specifications. These challenges may include inference between and within transmission points (TP), compatibility between release versions, increase demands of signal overheads and system capacities. In particular, various proposals have been made to improve the design of the channel state information reference signal (CSI-RS) and the download modulation reference signal (DMRS).
The channel state information reference signals (CSI-RS) are specifically intended to be used by a user equipment (UE) to acquire channel-state information (CSI) in the case when demodulation reference signals are used for channel estimation and downlink data demodulation. FIG. 1 illustrates, for instance, release 10 CSI-RS patterns with different CSI-RS configurations for 2, 4, and 8 antenna ports when normal cycle prefixes (NPC) are used. The numbering indices on the painted squares stand for the CSI-RS configuration indices which indicate the locations of CSI-RS signals on resource elements for each different CSI-RS configuration, and 20, 10, and 5 CS-RS configurations exist for 2, 4, and 8 CSI-RS ports respectively. Also please note that antenna ports 15 and 16 are used when there are 2 CSI-RS ports; antenna ports 15-18 are used in case of 4 CSI-RS ports; antenna ports 15-22 are used for 8 CSI-RS ports.
More specifically, the CSI-RS for each antenna pair could be allocated to the same resource elements but separated by using an orthogonal cover code (OCC) over two contiguous OFDM symbols. In other words, the division of the CSI-RS is accomplished by using the technique of code division multiplexing (CDM) expanded within 2 OFDM symbols in time domain and is called the CDM-T scheme. FIG. 2 shows an example of the CDM-T scheme for 8 antenna ports using the CSI-RS configuration #0 for time domain duplex (TDD) with 10 MHz bandwidth in case of NCP. The indices on the painted squares are the CSI-RS antenna port indices. Please note that the CSI-RS for the antenna pair 15 and 16 are allocated to the same resource elements. Likewise, the antenna pair 17 and 18, the antenna pair 19 and 20, and the antenna pair 21 and 22 also are allocated to the same resource elements.
An example of the mapping of the CSI-RS reference to specific resource elements could be seen according to Section 6.10.5.2 of the 3GPP technical specification TS 36.211 version 10.4.0 Release 10 which is hereby incorporated by reference. In this example, table 1 and table 2 together defines the allocation of CSI-RS resource as table 1 illustrates the mapping from each release 10 CSI-RS configuration to the specific resource element (k′, l′) for NCP where k′ is the subcarrier index and l′ is the OFDM symbol index, and table 2 illustrates the release 10 CSI-RS sub-frame configurations.
TABLE 1Mapping from Rel-10 CSI-RS configuration to the RE (k′, l′) for NCPNumber of CSI reference signals configuredCSI248Configuration(k′, l′)ns mod 2(k′, l′)ns mod 2(k′, l′)ns mod 2Frame structure type 10(9, 5)0(9, 5)0(9, 5)0and 21(11, 2) 1(11, 2) 1(11, 2) 1(FDD/TDD)2(9, 2)1(9, 2)1(9, 2)13(7, 2)1(7, 2)1(7, 2)14(9, 5)1(9, 5)1(9, 5)15(8, 5)0(8, 5)06(10, 2) 1(10, 2) 17(8, 2)1(8, 2)18(6, 2)1(6, 2)19(8, 5)1(8, 5)110(3, 5)011(2, 5)012(5, 2)113(4, 2)114(3, 2)115(2, 2)116(1, 2)117(0, 2)118(3, 5)119(2, 5)1Frame structure type 220(11, 1) 1(11, 1) 1(11, 1) 1only21(9, 1)1(9, 1)1(9, 1)1(TDD)22(7, 1)1(7, 1)1(7, 1)123(10, 1) 1(10, 1) 124(8, 1)1(8, 1)125(6, 1)1(6, 1)126(5, 1)127(4, 1)128(3, 1)129(2, 1)130(1, 1)131(0, 1)1
TABLE 2Rel-10 CSI-RS subframe configurationCSI-RS-CSI-RSCSI-RSSubframeConfigperiodicitysubframe offsetICSI-RSTCSI-RS (subframes)ΔCSI-RS (subframes)0-45ICSI-RS 5-1410ICSI-RS - 5 15-3420ICSI-RS - 1535-7440ICSI-RS - 35 75-15480ICSI-RS - 75
Section 6.10.5.1 shows an example of a CSI-RS sequence generation which could be expressed as follows:
                                          r                          l              ,                              n                s                                              ⁡                      (            m            )                          =                                            1                              2                                      ⁢                          (                              1                -                                  2                  ·                                      c                    ⁡                                          (                                              2                        ⁢                        m                                            )                                                                                  )                                +                                    j              ·                              1                                  2                                                      ⁢                          (                                                                    1                    -                                                                  2                        ·                                                  c                          ⁡                                                      (                                                                                          2                                ⁢                                m                                                            +                              1                                                        )                                                                                              ⁢                                                                                          ⁢                      m                                                        =                  0                                ,                1                ,                …                ⁢                                                                  ,                                                      N                    RB                                          max                      ,                      DL                                                        -                  1                                                                                        (        1        )            where ns is the slot index within a radio frame and l is OFDM symbol index within the slot. NRBmax, DL is the maximum number of resource blocks (RBs) for DL, and c(i) is a pseudo-random sequence and is initialized at the start of each OFDM symbol according to:cinit=210·(7·(ns+1)+l+1)·(2·X+1)+2·X+NCP  (2)where NCP=1 for NCP and 0 for extended CP (ECP) and X is NIDcell, which is the cell ID of the serving cell or transmit point (TP) and can be any one of the values from 0 to 503. An example of c(i) could be the length-31 Gold pseudo-random sequence as defined in section 7.2 of TS 26.211 v10.4.0. It should be noted that, the orthogonality of CSI-RS between cells/TPs could be achieved by using the same sequence initialization (i.e. ns, l, X and NCP) but different ports through cover codes. This means that the parameters ns, l, X and NCP would be required to produce the same initialization value for the CSI-RS between different cells/TPS in order to maintain orthogonalities. For example, port 15 and port 16 are orthogonal if they use the same sequence initialization. However, if different sequence initializations but same port of CSI-RS are used for the cells/TPs, the orthogonality of CSI-RS between cells/TPs will no longer hold.
The consequence of the need for orthogonalities is that extra reference signal overhead is required in the case of CoMP. For example, CSI-RS of 3GPP release 10 assumes a nesting property which means that a smaller number of CSI-RS ports would be a sub-set of a larger number of CSI-RS ports. Take FIG. 1 as an example, a 4-port CSI-RS would be a sub-set of an 8-port CSI-RS so that one 8-port CSI-RS resource can be shared between two cells/TPs as each of which can use one of the sub-sets of the 8-port CSI-RS. In other words, the two 4-port CSI-RS resources within the same 8-port CSI-RS resource, called as an intra-CSI-RS resource, can be used for the two cells/TPs as depicted in FIG. 3. It can be seen from FIG. 3 that two 4-port CSI-RS resources, configuration #2 and configuration #7, within the same 8-port CSI-RS resource can be respectively used for the Rel-11 or advanced version by UEs in cell #1 and cell #2 for joint transmission (JT) in CoMP with separate cell ID scenarios such as CoMP Scenario 3.
If cell #1 and cell #2 were in the CoMP scenario, according to the Rel-10 CSI-RS definition, the two 4-port CSI-RS resources within the same 8-port CSI-RS port would be required to apply the same scrambling sequence initialization generated based on for example cell #1. Under this circumstance, the 4-port CSI-RS resource using configuration #7 will no longer be used for the Rel-10 UEs in cell #2 due to mismatch of scrambling sequence initialization. In other words, the antenna ports for configuration #2 and #7 could no longer be assigned to the same resource elements using OCC since by doing so equation (2) would produce a different sequence initialization as the cell #2 uses a different set of parameters. Therefore, an additional 4-port CSI-RS resource with a corresponding muting scrambled based on cell ID #2, configured as configuration #4 for example, is needed for the Rel-10 UEs in cell #2 so that consequently an additional CSI-RS resource overhead which would have the same sequence initialization as cell #1 would be needed.
Based on FIG. 1, Table 1 and Table 2, it can be seen that the number of CSI-RS resources which could be configured within a subframe without collision is not by any means limitless. FIG. 4 shows that the increases of CSI-RS overhead would exacerbate in the case of 8-port CSI-RS resource for 2 TPs in the JT-CoMP scenario. Hence, the available CSI-RS resources would at some point be insufficient as the number of CoMP cooperating sets grows to a large number. To order to solve the aforementioned problem, one simple solution is to independently configure a virtual cell ID X in the sequence initialization per CSI-RI port or Per TP so that the CSI-RS can be individually used per CSI-RS port (or per TPs).
Besides the orthogonalities of the CSI-RS, the orthogonalities of the downlink (DL) demodulation reference signal (DMRS) must also be considered. A DMRS is a UE specific reference signal used for channel estimation and demodulation of the downlink data. FIG. 5 shows an example of a DL DMRS pattern in the case of NCP for the release 9 and release 10 of the 3GPP specification which specifies that antenna ports 7 and 8 are used for 2 DMRS ports, antenna ports 7-10 are used for 4 DMRS ports, and antenna ports 7-14 are used for 8 DMRS ports. Similar to a CSI-RS, CDM-T scheme may also be applied to DL DMRS. For rank 1˜4 transmission, OCC with length of 2 is used for DMRS; for rank larger than 4 transmission, OCC with length of 4 is used for DMRS. More specifically, for rank 1˜4 transmission, each antenna pair of the DMRS ports are allocated at the same resource elements but separated by using the OCC over the contiguous 2 OFDM symbols as illustrated by FIG. 6. Furthermore, each pair of DMRS ports may be separated from another pair by using the frequency division multiplexing (FDM) scheme. Please note that the indices on the painted squares stand for the DMRS antenna port indices.
A DL DMRS sequence can be expressed by equation (3) as below:
                                          r            ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                  2                          ⁢                          m                                                )                                                                                            )                                      +                                          j                ·                                  1                                      2                                                              ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                                              2                            ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ⁢                                  ⁢                  m          =                      {                                                                                0                    ,                    1                    ,                    …                    ⁢                                                                                  ,                                                                  12                        ⁢                                                  N                          RB                                                      max                            ,                            DL                                                                                              -                      1                                                                                        NCP                                                                                                  0                    ,                    1                    ,                    …                    ⁢                                                                                  ,                                                                  16                        ⁢                                                  N                          RB                                                      max                            ,                            DL                                                                                              -                      1                                                                                        ECP                                                                                        (        3        )            
where NRBmax, DL is the maximum number of resource blocks (RBs) for DL. For equation (3), c(i) could be a pseudo-random sequence according to section 7.2 of TS 36.211 v10.4.10 and is initialized at the beginning of each subframe usingcinit=·((ns/2)+1)·(2·X+1)·216+nSCID  (4)where ns is the slot index within a radio frame, X=NIDcell is the cell ID of the serving cell/TP and nSCID=0 (the scrambling ID) unless specified otherwise. For a physical downlink shared channel (PDSCH) transmission on port 7 or 8, nSCID for downlink control information (DCI) format 2B is indicated by the scrambling ID field and for 2C is recited below based on the specification according to 3GPP TS 36.212 release 10 version 10.5.0 which is hereby incorporated by reference.
TABLE 3Antenna port(s), scrambling ID and number oflayers indication for Rel-10 DCI format 2C.One Codeword:Two Codewords:Codeword 0 enabled,Codeword 0 enabled,Codeword 1 disabledCodeword 1 enabledValueMessageValueMessage01 layer, port 7, nSCID = 002 layers, ports 7-8, nSCID = 011 layer, port 7, nSCID = 112 layers, ports 7-8, nSCID = 121 layer, port 8, nSCID = 023 layers, ports 7-931 layer, port 8, nSCID = 134 layers, ports 7-1042 layers, ports 7-845 layers, ports 7-1153 layers, ports 7-956 layers, ports 7-1264 layers, ports 7-1067 layers, ports 7-137Reserved78 layers, ports 7-14
The first 4 rows at the left hand side and the first 2 rows at the right hand side of Table 3 show that the DMRS is applied to the multi-user (MU) MIMO (MU-MIMO) and the rest is for the single-user (SU) MIMO (SU-MIMO) which can be illustrated in FIGS. 7(a)-7(d) where FIG. 7(a) illustrates SU-MIMO transmitting two layer streams of data containing DMRS through antenna ports 7 and 8, FIG. 7(b) illustrates co-scheduled MU-MIMO with each UE having one layer stream and the same scrambling ID, FIG. 7(c) illustrates 2 UEs co-scheduled with each having a different nSID and two layers of data, and FIG. 7(d) illustrates co-scheduled 4 UE MU-MIMO with each having a single layer of data stream and two of the UEs may have the same scrambling ID. It should be noted that different scrambling ID or nSCID would result in different initiation values of scrambling sequences.
Since orthogonality means that at least two different DMRS with the same scrambling sequence through different antenna ports, and quasi-orthogonalities means that two different DMRS would have different scrambling sequences, it can be seen from the left hand side of table 3 that DMRS between value 0 and 2 would be orthogonal from each other, and DMRS between values 1 and 3 would be orthogonal. DMRS between values 0 and 1 and between 2 and 3 would be quasi-orthogonal. As for the right hand side, DMRS between different layers of data stream within the same UE would be orthogonal but quasi-orthogonal between different UEs.
Therefore, in order to control orthogonalities of DMRS signals, the parameters nSCID, X, and ns must be carefully chosen to satisfy new requirements for further releases. Requirements for a future release has been suggested as (1) interference randomization between TPs, (2) orthogonal MU pairing within each TP, (3) backward compatible orthogonal MU pairing between releases, (4) orthogonal MU pairing across TP border, and (5) feasibility for Joint Transmission (JT) and dynamic point selection (DPS). At this point, it has been agreed that DL DMRS sequence would be further enhanced for a future release version in which the value of X can be dynamically chosen from {x(0), x(1), . . . , x(N−1)}, where N is an integer and 1≦N≦503, and x(n) (0≦n<N) are configured by UE specific radio resource (RCC) signaling. Currently there have been two solutions to dynamically select the parameter X.
The first solution is that X is dynamically selected from x(0) and x(1), and nSCID in DCI is re-used for dynamic selections of x(0) and x(1) according to equation (5):
                    X        =                  {                                                                      x                  ⁡                                      (                    0                    )                                                                                                                    n                    SCID                                    =                  0                                                                                                      x                  ⁡                                      (                    1                    )                                                                                                                    n                    SCID                                    =                  1                                                                                        (        5        )            According to equation (5), X is dynamically selected and is tied to nSCID. Solution 1 is summarized according to table 4, where NIDserving is the ID number of the serving TP, and NIDcommon could be the ID number of the macro cell or an additional virtual ID. Solution 1 thus can support two dynamic DL DMRS sequences per TP configured for UEs.
TABLE 4nSCIDx(n) = x(nSCID)Antenna port, v0x(0) = NIDserving781x(1) = NIDcommon78
The second solution is that X is dynamically selected from x(0) and x(1) using an additional bits in DCI. Solution 2 thus supports 4 dynamic DL DMRS sequences configured for UE with two bits supporting four combinations. Solution 2 is summarized by table 5 below:
TABLE 5nSCIDx(n)Antenna port, v0x(0) = NIDserving78x(1) = NIDcommon781x(0) = NIDserving78x(1) = NIDcommon78Solution 1 is currently considered the solution since it has less reference signal overhead and less impact on standardization.
FIG. 8A-8E illustrates the impact of Solution 1 mentioned above on CoMP scenario 3. Requirement (1) requires that interferences be randomized between TPs, and requirement (2) requires orthogonal pairing within each TP. In order to achieve requirement (1), co-schedule UEs within different TPs must be configured with different DL DMRS scrambling sequence initialization values so as to be quasi-orthogonal between TPs as illustrated by FIG. 8A. This implies that different values of X or different values of nSCID should be configured from one TP to another. In order to achieve requirement (2), same values of nSCID should be configured within each TPs.
Requirement (3) requires orthogonal MU pairing between Rel-10 EUs and Rel-11 UEs. To achieve requirement (3), co-scheduled UEs are configured with same DL DMRS scrambling sequence initialization value which meaning that the same values of nSCID and X should be configured among the co-scheduled UEs as illustrated by FIG. 8B. Requirement (4) requires orthogonal MU pairing across TP border. To achieve requirement 4, co-scheduled UEs are configured with same DL DMRS scrambling sequence initialization value which means that the same value of nSCID and X should be configured for the co-scheduled UEs as illustrated in 8C.
Require (5) requires feasibility for JT and DPS. To achieve JT for Requirement 5, the co-scheduled UEs are needed to be configured with same DL DMRS scrambling sequence initialization value which means that the same value of nSCID and X should be configured for the co-scheduled UEs as illustrated in FIG. 8D. To achieve DPS for Requirement 5, the UE is configured with different DL DMRS scrambling sequence initialization values for different TPs which means that different values of X or different values of nSCID should be configured for the UE for different TPs as illustrated in FIG. 8E.
It should be noted that as far as the enhancement for sequence initialization is concerned thus far, the virtual ID X has received the most attention. However, the slot number ns has not been taken into consideration. Under CoMP scenarios when the cooperating cells/TPs are not synchronous, differences in the slot number ns among each cell/TP would have an adverse impact on the sequence initialization for both the CSI-RS and DL DMRS, and therefore, a design of sequence initialization using the slot number ns is needed.